回复: [PATCH v4 4/4] PCI: mediatek-gen3: Add Airoha EN7581 support

[Bjorn Helgaas]

On Fri, Nov 08, 2024 at 01:23:35AM +0000, Hui Ma (马慧) wrote:
> > On Thu, Nov 07, 2024 at 05:21:45PM +0100, Lorenzo Bianconi wrote:
> > > On Nov 07, Bjorn Helgaas wrote:
> > > > On Thu, Nov 07, 2024 at 08:39:43AM +0100, Lorenzo Bianconi wrote:
> > > > > > On Wed, Nov 06, 2024 at 11:40:28PM +0100, Lorenzo Bianconi wrote:
> > > > > > > > On Wed, Jul 03, 2024 at 06:12:44PM +0200, Lorenzo Bianconi wrote:
> > > > > > > > > Introduce support for Airoha EN7581 PCIe controller to 
> > > > > > > > > mediatek-gen3 PCIe controller driver.
> > > > > > > > > ...
> > > > 
> > > > > > > > Is this where PERST# is asserted?  If so, a comment to 
> > > > > > > > that effect would be helpful.  Where is PERST# deasserted?  
> > > > > > > > Where are the required delays before deassert done?
> > > > > > > 
> > > > > > > I can add a comment in en7581_pci_enable() describing the 
> > > > > > > PERST issue for EN7581. Please note we have a 250ms delay in 
> > > > > > > en7581_pci_enable() after configuring REG_PCI_CONTROL register.
> > > > > > > 
> > > > > > > https://github.com/torvalds/linux/blob/master/drivers/clk/cl
> > > > > > > k-en7523.c#L396
> > > > > > 
> > > > > > Does that 250ms delay correspond to a PCIe mandatory delay, 
> > > > > > e.g., something like PCIE_T_PVPERL_MS?  I think it would be 
> > > > > > nice to have the required PCI delays in this driver if 
> > > > > > possible so it's easy to verify that they are all covered.
> > > > > 
> > > > > IIRC I just used the delay value used in the vendor sdk. I
> > > > > do not have a strong opinion about it but I guess if we move
> > > > > it in the pcie-mediatek-gen3 driver, we will need to add it
> > > > > in each driver where this clock is used. What do you think?
> > > > 
> > > > I don't know what the 250ms delay is for.  If it is for a required 
> > > > PCI delay, we should use the relevant standard #define for it, and 
> > > > it should be in the PCI controller driver.  Otherwise it's 
> > > > impossible to verify that all the drivers are doing the correct delays.
> > > 
> > > ack, fine to me. Do you prefer to keep 250ms after 
> > > clk_bulk_prepare_enable() in mtk_pcie_en7581_power_up() or just use PCIE_T_PVPERL_MS (100)?
> > > I can check if 100ms works properly.
> > 
> > It's not clear to me where the relevant events are for these chips.
> > 
> > Do you have access to the PCIe CEM spec?  The diagram in r6.0, sec 
> > 2.2.1, is helpful.  It shows the required timings for Power Stable, 
> > REFCLK Stable, PERST# deassert, etc.
> > 
> > Per sec 2.11.2, PERST# must be asserted for at least 100us (T_PERST), 
> > PERST# must be asserted for at least 100ms after Power Stable 
> > (T_PVPERL), and PERST# must be asserted for at least 100us after 
> > REFCLK Stable.
> > 
> > It would be helpful if we could tell by reading the source where some 
> > of these critical events happen, and that the relevant delays are 
> > there.  For example, if PERST# is asserted/deasserted by 
> > "clk_enable()" or similar, it's not at all obvious from the code, so 
> > we should have a comment to that effect.
> 
> >I reviewed the vendor sdk and it just do something like in clk_enable():
> >
> >	...
> >	val = readl(0x88);
> >	writel(val | BIT(16) | BIT(29) | BIT(26), 0x88);
> >	/*wait link up*/
> >	mdelay(1000);
> >	...
> >
> >@Hui.Ma: is it fine use msleep(100) (so PCIE_T_PVPERL_MS) instead
> >of msleep(1000) (so PCIE_LINK_RETRAIN_TIMEOUT_MS)?
>
> 	I think msleep(1000) will be safer, because some device won't
> 	link up with msleep(100).

Do you have details about this?  I guess it only hurts mediatek, but
increasing the minimum time to bring up a PCI hierarchy by almost an
entire second is a pretty big deal.

If this delay corresponds to the required T_PVPERL delay and 100ms
isn't enough for some endpoints, those endpoints should fail with many
host controllers, not just mediatek, so I would suspect the mediatek
controller or a certain platform, not the endpoint itself.

If this corresponds to T_PVPERL and mediatek needs longer, I would
document that by using "PCIE_T_PVPERL_MS * 10" and adding a comment
about why (affected platform/device, hardware erratum, etc).

Bottom line, I don't really care what the value is, but I *would* like
to be able to read pcie-mediatek-gen3.c and see the point where PCI
power is stable, a delay of at least T_PVPERL, and where PERST# is
deasserted because that's the main timing requirement on software.

Bjorn